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This video separates science from the pseudo-science regards floods and extreme precipitation in part 7 of how pressure systems control climate. Floods are the most frequent of all natural disasters. The deadliest of all recorded floods devastated regions of china in cooler times of the late 1800s and 1930s. But the visible heartache of floods today makes them lucrative click-bait for mainstream media and good optics to push a climate crisis. For example, last year national public radio promoted pseudo-science in order to frame floods as a new existential threat.
Level ground, rich soils and easy access to water have enticed people to colonize flood plains for millennia, despite the risk of inevitable catastrophic floods. Building levees was one solution. But by denying a river access to its natural floodplains, levees amplified downstream flows only to re-locate flooding. This video won’t delve into the effects of human levee systems, here I focus on the latest science to establish how much flooding can be expected from natural climate variability. Due to the inevitable natural frequency of floods, by 1920 private insurance companies in America stopped offering flood insurance.
Unwisely, governments then offered flood insurance that has unintentionally encouraged people to keep staying on the flood plains, no matter how often they needed to rebuild, and no matter how much wetland ecosystems are lost. As seen here, Florida’s St John’s River lost over 90% of its floodplains to encroaching humanity in 70 years. But observing the environmental damage there are now ongoing efforts to restore as much wetlands as possible.
With the advent of the satellite era, scientists have been able to construct a much more accurate picture of the earth's precipitation patterns and flood risks from climate change.
Kevin Trenberth, from the national center for atmospheric research, is an outspoken scientist who has promoted much pseudo-science regards floods, and unfortunately, he's usually the first scientist mainstream media interviews in search of a climate crisis headline.
He generates pseudo-science by mis-applying the proven Clausius-Clayperon equation, which states a 1-degree Celsius rise in temperature increases the atmosphere's moisture holding capacity by 7%, arguing theoretically that global warming must be increasing rainfall and making floods worse. But while the Clausius-Clayperon equation is verifiable in a laboratory setting. It doesn’t explain global precipitation patterns.
Nguyen's 2018 satellite-based examination of precipitation shows the average rates of regional precipitation around the globe. The trade winds carry moisture evaporated from the relatively cloudless regions represented here in dark blue and concentrate that moisture in the intertropical convergence zone, or ITCZ. The ITCZ is represented here by the darker red colors circumscribing the equator. The ITCZ accounts for about 32% of all global rainfall and the importance of a shifting ITCZ on wet and dry climates was discussed in earlier parts of this series. If global warming has increased evaporation, then the ITCZ would be one of the most sensitive regions to support Trenberth’s proclamations that global warming is causing a climate crisis and more extreme rainfall.
The western pacific warm pool is the largest ITCZ region of heavy rainfall, yet in the heart of the warm pool, Indonesia’s elevation differences illustrate how interactions with cold temperatures greatly affect the degree of rainfall. In Indonesia’s lowlands receive 70 to 125 inches of rain fall while Indonesia’s higher and cooler elevations, 2 to 3 times as much rain fall.
Clearly illustrated here warmer temperatures simply don’t translate to more rainfall, over the Sahara Desert where maximum temperatures average 40 degrees Celsius or 104 Fahrenheit, there is only enough moisture to produce 3 inches of rain each year. In contrast, Dublin Ireland receives over 10 times that amount of rainfall despite a much lower average high temperature of just 15 degrees Celsius or 59 Fahrenheit.
Clearly atmospheric circulation can trump the Clausius-Clayperon temperature equation.
Using 33 years of NOAA’S satellite data, Nguyen 2018 also mapped the world's trends in precipitation. Revealing a vast mosaic of increasing and decreasing trends that defies Trenberth’s pseudo-science. The blue areas represent increasing rainfall trends of which only 2.3% of the earth experiences any statistically significant increasing trend. Regions of decreasing precipitation are illustrated by red colors, and cover half the earth, but significant drying covers twice the area of significantly increasing precipitation. To see more clearly where statistically significant changes are happening, Nguyen 2018 produces this map.
And contrary to Trenberth’s claim that global warming will cause convergence zones of moisture to exhibit increased precipitation, over 95% of the ITCZ region has shown no significant change in 30 years casting serious doubt on the usefulness of either the Clausius-Clayperon equation or global warming average statistic. Likewise, over the USA where extreme precipitation events and floods are quite common, there has been no significantly increasing rainfall.
In 2011, Michael Dettinger a hydrologist with the us geological survey and a Scripps researcher, published this illustration showing the location of weather stations reporting extreme rainfall of 400 mm or 15.8 inches or more of rainfall over 3-day periods spanning the years 1950 to 2008.
It is well known that most extreme rainfall is largely associated with hurricanes and atmospheric rivers. Accordingly extreme rainfall is observed along the Gulf coast and eastern coast of the USA during the warm months, when and where hurricanes are most impactful.
Purple dots represent weather stations that have recorded one extreme event in the past 60 years while the blue dots represent locations observing 2 or three extreme events during that time span.
Atmospheric rivers deliver the extreme rainfall events in California happening mostly during the cooler months of late fall, winter, and early spring. There, a few weather stations have recorded extreme precipitation 6 and 7 times in 60 years.
The water cycle helps illuminate the source of moisture for various extreme rainfall events. 85% of the earth's moisture evaporation occurs over the ocean, but only 90% of that moisture falls back harmlessly over the ocean.
The missing 40,000 cubic kilometers of moisture is transported from the oceans to the land each year, but it only accounts for 35% of all the rainfall on land. That's because 65% of continental precipitation is fueled by recycled moisture via evaporation from lakes and wetlands and transpiration by vegetation.
Because the eastern USA has more wetlands and more forests than the west, it recycles more rainfall, During the warm months of June and July about 60% of the eastern USA’s rainfall is sourced from recycled moisture. So, the eastern USA should also be a sensitive indicator of any global warming induced evaporation and rainfall as suggested by Trenberth.
But in contrast to Trenberth’s theoretical pseudo-science, Nguyen’s 2018 analyses show The eastern USA has not experienced any precipitation trends in 30 years.
Depending on what model is used, results often differ. Nevertheless Kazmzadeh's 2021 satellite analysis, likewise, found no precipitation trends in the eastern USA for the most recent 20 years.
Using a different time frame, Kazemzadeh did find significant trends in different locations than Nguyen. But still, only 6.1 % of the earth experienced any statistically significant increased precipitation while 6.1% experienced significant decreases. Both studies suggest no change in the supply of moisture for rainfall. And in contrast to Trenberth’s expectation of amplified precipitation where moisture converges, hardly any portion of the ITCZ’s region experiences any increased rainfall.
In addition to the effect of different starting dates for rainfall trends, how a region's boundary is defined also alters trend analyses. Because state agencies manage water resources, Nguyen 2018 analyzed rainfall trends by political boundaries, resulting in 10 states exhibiting insignificant trends of increasing rainfall while 5 states experienced significant decreasing trends.
A much different picture arose when defining regions by the more climatically meaningful watershed boundaries. Such analysis then found no regions of the USA experiencing any increase in rainfall, but a significant drying trend in the Colorado river basin.
So beware, depending on a researcher's agenda, they can cherry-pick the statistics that best suits their narrative.
If Trenberth’s theoretical proclamations are true that global warming has increased extreme rainfall, it should be reflected in higher flood peaks in the rivers. To test the global warming claims, Gabrielle Villarini examined 50 long-term stream gauge stations, each with a 100+ years of data. As her results here indicate, there has been no increasing trend in peak river flows. The range of flow volumes remained stationary within the bounds of natural variability.
Nonetheless, there are always outlier years with damaging floods and those years provide the misleading click-bait headlines for the media. The oldest USA station that has monitored the Connecticut river since 1836, had a major flood due to the rains of the march hurricane of 1936. So damaging was that flood, it prompted the USA’s flood control act of 1936.
Given the lack of any long-term river flow trends, Villarini concluded
"it is easier to proclaim the demise of stationarity of flood peaks than to prove it through analyses of annual flood peak data."
The causes of flood damage from hurricanes are more complex and must consider extreme precipitation, coastal surges, coastal landscape changes and broken levees.
But as expected the devastating flood from hurricane Katrina in 2005 provided Trenberth with the optics to push his single-minded obsession with a climate crisis. Again, he repeated that the Clausius-Clayperon equation dictated worse floods due to an 8% increase in extreme precipitation and a warmer ocean making Katrina stronger.
But Katrina evolved as local conditions changed, not as a global warming statistic changed. Katrina was a small category 1 hurricane when it first hit Florida, then evolving into a category 5 over the Gulf of Mexico but declining into a category 4 and 3 as it hit the gulf's coastline.
If we cherry pick Katrina’s time as a category 5 hurricane, it rates as the 12th strongest in 150 years. But there were stronger hurricanes in the 1930s and 50s, contradicting claims that global warming had caused stronger hurricanes.
The horrific flooding was not due to human effects on climate but, how humans had degraded the landscape around New Orleans. By altering the Mississippi river's course, and draining and urbanizing the wetlands of its floodplains, parts of New Orleans, are now 3 to 5 meters below sea level. Even without a hurricane, any failure of its levees, would produce a devastating flood. A recent study by Dixon in 2005 found New Orleans is still sinking at a rate of 6.4 mm/year and as much as 33 mm/year. Sinking land is a far bigger threat than rising sea level.
Where hurricanes make landfall is primarily determined by how the winds from the Atlantic subtropical high-pressure system guides the storm. Known as the Bermuda high when centered more towards the USA, the winds more often drive hurricanes into the Gulf of Mexico, relative to times when the high is centered further east and known as the azure high, which causes more hurricanes to pass up the middle of the Atlantic with little coastal impacts.
This relationship with the high-pressure system is clearly seen by mapping the frequencies of hurricane storm tracks. As represented by the dark red regions, hurricanes more frequently pass harmlessly northward much to the east of the USA. The 2 regions that experience the most landfalling hurricanes are around New Orleans and the east coast from Florida to North Carolina when the high moves westward.
Climate scientist have been deeply divided on whether or not global warming is causing more Atlantic hurricanes. Using raw data, Trenberth’s ilk point to a rising trend as seen in green. Other scientists argue before the satellite era, many hurricanes went undetected and thus were underestimated unless they came closer to shore. So those scientists adjust the data and find no trend as illustrated by the orange line.
Hurricane landfall data is more robust, and it too finds no increasing trend.
And accordingly, there is no significant trend in precipitation in the USA where it is most affected by hurricanes.
High precipitation in every cyclonic storm, whether called hurricanes, extratropical cyclones or atmospheric rivers is primarily due to warm moisture transported poleward from the tropics by the warm conveyor belt As the warm conveyor approaches colder air to the north, it rises and cools, causing moisture to condense and rain out. The rising air of the warm conveyor also induces cold dry air to sink from the upper atmosphere to maintain a mass balance.
These dynamics of all cyclonic storms are readily seen in satellite photos. The long gold arrow illustrates the path of the warm conveyor that is causing clouds to form as it rises. Also note the faint outline of the USA showing the moisture is being pulled from the tropics south of the Yucatan The dark band paralleling the warm conveyor represents the cold dry air that has descended into the cyclone.
The earth's more frequent atmospheric rivers transport more moisture poleward than hurricanes, likewise, via their warm conveyor belts, they transport more than 90% of all tropical moisture reaching the mid latitudes.
Globally about 130 atmospheric rivers occur each year with 20–30-hour lifespans. Disproportionately, California averages 15 a year, explaining why California is such a hot spot for extreme precipitation. Globally there can be 5 or 6 atmospheric rivers happening at any one time, but not all make landfall.
Like hurricanes, the path of an atmospheric river is determined by the configuration of high and low-pressure systems.
Here an atmospheric river extends poleward from the pacific warm pool With a contracted low-pressure system centered around the Aleutian Islands, the atmospheric river was guided north of san Francisco (the green dot) and into British Columbia on September 21, 2021.
One month later, the low-pressure system had expanded and moved southward. Along with the high-pressure system to the south the atmospheric river was guided into California.
San Francisco received over 4 inches of rain in a single day, ranking as the 4th most ever recorded. But we can’t blame global warming More and stronger rainfalls happened in the cooler 1800s.
The great flood of 1862 was caused by an atmospheric river’s rain on snow event in the sierra Nevada that resulted in the downstream flooding of Sacramento. By studying the amounts of sediments delivered by heavy river flows into the Santa Barbara basin and san Francisco Bay, scientist have identified floods caused by past atmospheric rivers. During the cooler times of the Little Ice Age between 1300 and 1860s ad, California experienced several atmospheric river-induced megafloods. The greatest of all in over 2000 years happened in 1605.
So why so many mega-floods during the cooler little ice age?
Although debated, most researchers have determined that during the little ice age the Pacific Ocean existed mostly in an el Nino-like condition with more rainfall over the eastern pacific due to warmer surface temperatures In combination with more atmospheric rivers initiating from a warmer eastern pacific, the research by Zhou 2019 shows El Nino-like conditions configure the cyclonic low-pressure systems and anticyclonic high-pressure systems to drive more atmospheric rivers into California
During La Nina-like conditions, the eastern Pacific is cooler and the western pacific is warmer. This promotes more atmospheric rivers beginning in the western pacific warm pool.
In combination with the resulting changes in the pressure systems, more atmospheric rivers are guided northward into British Columbia and California becomes drier.
During La Ninas the resulting warming of the western pacific warm pool also promotes more atmospheric rivers into the southern hemisphere, One which recently flooded Australia’s Brisbane region. The configuration of pressure systems focused that atmospheric river onto Brisbane and its surrounding regions, bringing a record 24.1 inches of rain in just 3 days
But that is not the record rainfall for Brisbane’s watershed.
As has been the case so often, It was during cooler times, that a stronger atmospheric river inundated the region. In 1893, In the nearby town of Crohamhurst a record 35.7 inches of rain fell in just one day.
So, make no mistake. As experts have warned, megafloods are coming, with the same devastating force as they have in the past. Climate models predict worse atmospheric rivers due to increased evaporation and the atmosphere’s greater moisture capacity from global warming, but satellite data contradicts those claims, making forecasts based on global warming useless.
And data from the little ice age shows atmospheric rivers are independent of global warming. However, on the good side, forecasting the location of devastating floods maybe more predictable because the path of hurricanes and atmospheric rivers are, modulated by natural oscillations and their effect on observable guiding pressure systems, But despite better weather forecasting, the real worry is people continue to colonize more and more flood plains, putting themselves in harm’s way.
If you live in a flood plain, cutting your co2 emissions won't stop the floods. The wisest plan is to move to higher ground or else keep reinforcing your levees.